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Long-term monitoring of the recurrent nova U Scorpii

Update: 2010 April 9

The following update was sent by Brad Schaefer on April 9, 2010 to participants in the global observing campaign on U Sco.

Here is a short update on the U Sco eruption as of now (Day D+71).

From Day D+11 to D+30, U Sco was in a plateau, followed by a moderately sharp drop in all bands. A second plateau was visible in the light curve from D+41 to D+57 at V=16.8+-0.2 (outside of eclipses). This plateau ended with a fairly sharp drop from D+57 to D+64, where the out-of eclipse brightness came to the V=18.0 mag level. This level is the same as the long and short term average for the pre-eruption light. That is, on Day D+64, the U Sco eruption ended.

The time from peak to quiescence was 64 days, which is the all-time fastest. The nearest competitors are the eruptions of the recurrent novae RS Oph and T CrB, both coming in at 93 days.

This eruption of U Sco is now the all-time best observed nova event. We have high-time-density observations of both spectroscopy and photometry in all of the X-ray, UV, optical, and IR bands. (U Sco was not detected in the radio or the gamma-ray ranges, and we only have a few days worth of data from 3.2 to 33 microns.) This is the first nova that we have daily spectral energy distributions from X-ray to IR throughout the eruption. The photometrists of the world have kept the nova under surveillance with fast photometry for the majority of time throughout the 61 days of the eruption, with 22,000 magnitudes already accumulated. This wonderful coverage of all types is an awesome observational triumph from roughly 100 observers around the world. (Each of six continents has observers featured prominently for various tasks, and we have data from 9 satellites also.) I have been often impressed with the many people and groups going far beyond the call of duty so as to get the very best of data. We should all have celebrations of the fantastic job and the wonderful data sets accumulated.

Many mysteries have arisen from this eruption. For example, what caused the triple peak optical & IR line profiles (the 'Batman cowl' shape) that lasted for a few days after the peak? Also, what is U Sco doing with *two* flat plateaus? Another puzzling example is that U Sco displayed short (~half-hour duration) flares with amplitudes up to half a magnitude along with continual variations on half-an-hour timescale in the first 11 days, over times when the nova shell was optically thick so we are seeing shell phenomena. (For this last point, the only reason that we have discovered this new phenomena is that this is the first time that anyone has ever put fast photometry on a nova near peak. [If anyone knows of a prior case with substantial amounts of fast photometry, then please let me know.] This also suggests that future novae should have fast photometry so as to define the extent of the phenomenon.) And my analysis of the eclipse timings during the tail confirms the period change across the prior nova event that I got from quiescent observations, so we are left with the horrifying problem of explaining how the orbital period can *decrease* suddenly across the eruption.

The real and unique science output from all this wonderful data will be when large data sets are consistently analyzed together. Here are some of the big efforts that I know are underway around the world: (1) All the spectra, from X-ray to UV to optical to IR are being collected together for one big global analysis where everything is being put together in a consistent way. This global analysis will yield a measure of the mass ejected, the elemental abundances in the ejecta, and the physical conditions throughout the shell throughout the eruption. (2) For the first time ever, we have the full spectral energy distribution on every day of an eruption, and this is being put together to measure the total energy radiated by the nova. This quantity is directly proportional to the mass ejected, giving us a second method for this quantity of such high importance for determining whether U Sco is a progenitor of Type Ia supernovae. (3) When combined with old data, the eclipse times will give the orbital period change across eruptions. The goal was to use this period change to determine the mass ejected by yet another means, but this final calculation will apparently not be possible for U Sco, even though it leaves us with a bigger mystery. (4) We have wonderful photometry and spectroscopy across many eclipses throughout the eruption, with this providing a unique capability of eclipse mapping the nova SSS and inner photosphere. We will be able to map out the brightness and temperature of both the X-ray-to-IR continum but also of many line-producing regions. (5) We have fantastic amounts of data with flickering and flares throughout the eruption with much data simultaneous across the bands from X-ray to optical. This will yield the time scales, size distributions, and spectral energy distributions of the flare light, and hopefully answer the mystery of their cause.

Our community has only done 'half' the work so far. After taking the all-time best data set for a nova, we have to fully reduce the data and make timely publications. And, importantly, theorists will then have to work on models that should be incredibly detailed. With this eruption, every observational question can be answered. Every theorist with a model of any aspect of nova eruptions must confront their models with the U Sco 2010 eruption data.

We have a decade for celebrations and being proud of all our contributions to this historic event in nova studies. And in the year 2020+-2, we can expect another U Sco eruption!

Cheers, Brad

Update: 2010 March 11

We are now six weeks (42 days) since the eruption of U Sco on 2010 January 28, and the AAVSO's international observer community has collaboratively created a light curve with over ten thousand measures of U Sco, one of the best-observed novae in history. The response of both the amateur community and the general astronomical community has been amazing to see, and as someone with feet in both the professional and amateur communities, it's been gratifying to see the good science that has arisen from the AAVSO's participation in Bradley Schaefer's global campaign. I'm looking forward to the new astrophysics that comes from the analyses of these data, and to seeing the work of AAVSO observers playing a major role in that.

As of today, 2010 March 11, U Sco is now around V=17 out of eclipse, just a magnitude above its quiescent level. Observations will continue for quite some time, even after the 2010 eruption is considered "over", and the AAVSO observers may continue to play a part in that. Future photometry of the eclipses of U Sco may help researchers to derive exactly how much mass the system lost during the current eruption, as well as provide more clues about the shape of the system. Likewise, continued photmetry of the star in quiescence (particularly in B) will help monitor the rate of mass transfer, and hence help serve as a predictor of exactly when the next eruption is coming. As with all variables in the AAVSO observing program, more observations are needed, and are most definitely encouraged! We're still very early in U Sco's observing season, and observers are encouraged to continue monitoring this star throughout the coming year and beyond.

The following observers have contributed observations to the AAVSO as part of this observing campaign, and we offer our sincere appreciation to each and every observer:

On behalf of the AAVSO, I would like to extend my thanks to everyone who contributed to the great success of this observing campaign, with particular thanks to Barbara Harris and Shawn Dvorak for their quick work in announcing their discovery!

Clear skies, and Good Observing!

Dr. Matthew Templeton
AAVSO Campaign Coordinator

Update: 2010 March 8

U Scorpii appears to be in decline again, with the last reported observations by Arto Oksanen putting U Sco at around CV=16.7. The star was predicted to undergo a rapid decline right around this time, so please follow U Sco as much as possible over the coming days and weeks.

U Sco is faint at quiescence, around V=18.2, so it is only about 1.5 magnitudes above its pre-outburst brightness. Try to find the best balance between signal-to-noise and short exposure times if you're performing CCD photometry. You should aim for a signal-to-noise of at least 10 when doing CCD photometry, but low signal to noise (e.g. S/N ~ 3) is acceptable to show the star was detected. And don't forget that visual observers can contribute too!

Update: 2010 March 3

The plateau phase of the U Sco outburst is showing signs of ending, and the star now appears to be around V=15.5. Dr. Brad Schaefer has requested continued intensive observations of U Sco, including time-series. In other exciting news, Dr. Jan-Uwe Ness of ESA has upcoming X-ray time-series photometry scheduled with the XMM-Newton satellite, which were the subject of AAVSO Special Notice #197 issued on March 2. Optical photometry obtained by ground-based observers will be compared to the X-ray time series to study the emission mechanisms for light at these two wavelengths. Doing so will shed light on the physics of this fascinating object.

We're now more than a month into U Sco 2010, and this outburst is shaping up to be one of the best observed outbursts of any recurrent novae in history. The partnership between amateur and professional astronomers has resulted in a multiwavelength dataset that will be useful for researchers for decades to come, and it was all made possible by the hard work and dedication of observers like you!

Brad Schaefer has written and posted a comprehensive update on the progress of this outburst, which has now been posted to the AAVSO Discussion mailing list. Click here to read it!

On behalf of the AAVSO and of the scientific community, thank you for your continued observations of U Sco, and of all of the other variable stars! Way to go!

Update: 2010 March 1

The U Sco outburst continues. Now is an important time to observe since the star will likely begin a rapid fade within the next few weeks. Please continue to observe U Sco, and if it begins to fade, please report your observations as soon as you can!

Update: 2010 February 22

U Scorpii remains at a plateau around V=14.5 although it may be starting to decline slightly. An Astronomer's Telegram on eclipses in U Scorpii was issued on 2010 February 20 by Dr. Bradley Schaefer, along with a number of AAVSO, CBA, and RASNZ-VSS observers. You can find it here: ATel #2452: Recurrent Nova U Sco Shows Deep Optical Eclipses During Plateau Phase. The data shows that the eclipses are very well defined now, and you can continue tracking the eclipses using this list of projected eclipse times. Again, all observations including visual, CCD time-series, and multicolor photometry are valuable. Thanks to everyone who has contributed observations so far!

Update: 2010 February 19

U Scorpii has been holding at a plateau around V=14.0 for several days now, but that doesn't mean it hasn't been doing interesting things! Flickering has been evident for a few weeks now, and well-defined eclipses have now appeared at all wavelengths. Amateur observers with the AAVSO, the Center for Backyard Astrophysics, and Variable Stars South (the RASNZ Variable Star Section) have been covering the light curve throughout the outburst, and observations are needed now more than ever. Please see this list of projected eclipse times to observe them, but don't forget we need photometry outside of eclipses too!

The response of the AAVSO and other variable star observing organizations to the U Sco outburst has been phenomenal; despite the fact that it remains an early morning object, the 2010 outburst of U Sco will be its best-observed outburst in history. Likewise, the professional community has turned telescopes operating at all wavelengths toward U Sco as well, and this outburst will be one of the best-observed (and perhaps best-understood) nova events of all time. All observers -- instrumental and visual -- are still needed, so please contribute your observations!

Update: 2010 February 10

U Sco has reached V=14.0, but appears to have slowed its decline temporarily. Now is the time to begin intensive observations! This outburst of U Sco will be the best-observed in history, and your contributions will play an important part in the scientific work to come.

Update: 2010 February 6

U Sco continues to decline and is below V=13.0 now, five magnitudes below maximum. According to IAU Circular 9114 (D.W.E. Green, editor), Worthers, Eyres, Rushton, and Schaefer have found that the star has begun to flicker again, which means the nova ejecta shell has become optically thin again, and we can see the central star. This means we'll also start to see eclipses again, and the combination of both flickering and eclipses means that time series observations are critically needed right now. If you are able to do time-series observations, please do!

Brad Schaefer sent an update on eclipse timings, and has requested that observers target these eclipses as best they can:

As always, all observations are needed, including visual. Now that the source is fading, it may soon be out of range of some observers if it is not already, but don't forget that fainter-thans are also valuable data!

Update: 2010 February 4

U Sco has declined by more than four magnitudes since the outburst began on January 28, only six days! It is still early in the U Sco observing season but we're getting good coverage so far. Thanks to everyone who is able to get out in the early morning hours and observe! We need both visual and CCD observers to participate -- all data are welcome.

We should begin to see changes in the behavior soon, including eclipses and color changes in the spectrum. If you're a CCD observer, please consider trying time-series if you have one or no filters. If you have multiple filters, try to track the color evolution of this nova in whatever filters you have available.

Update: 2010 January 29

The current eruption of U Scorpii is proceeding as previous events have done. In the V-band, the star has faded by more than one magnitude in a single day. Spectroscopy by observers in Japan and India has revealed similar spectroscopic features to those seen before, notably the very broad emission lines indicating a large expansion velocity for the ejected nova shell; the edges of the shell are travelling at 11,000 kilometers per second relative to the the star!

Visual and CCD observations of U Sco are strongly encouraged! This is a morning object but any loss of sleep is definitely worth it. If you have more than one filter, please try to take at least one set of multicolor observations per observing session in addition to any single-filter time-series.

For more scientific and historical background on U Sco, please see the Variable Star of the Season article on U Sco from July 2009.

Update: 2010 January 28

U Scorpii is in outburst!

The outburst was detected by Barbara G. Harris (HBB) of New Smyrna Beach, Florida on 2010 January 28.4385 UT (see image at right). The outburst was subsequently confirmed by Shawn Dvorak (DKS) of Clermont, Florida on January 28.47. A subsequent set of time-series by Dvorak suggests that U Scorpii was caught at or near maximum, prior to its expected rapid decline. This is the first outburst of U Sco since February 1999, and occurred within the time window of 2009.3 +/- 1.0 years predicted by Dr. Bradley Schaefer in 2005. The global campaign to monitor the U Sco eruption at all wavelengths has now begun!

Any and all observations of U Scorpii are critically needed right now. This includes visual observations, PEP and CCD observations in multiple filters, and time-series photometry. If you can observe U Sco, please do! The more observations the better!

For more information, please see AAVSO Alert Notice #415.

As of January 28 17:00 UT, both RXTE and INTEGRAL have scheduled observations of U Sco, and may have already acquired data! More updates will be forthcoming as the campaign continues.

This observing campaign was announced in Alert Notice 367

Dr. Bradley E. Schaefer (LSU) requests long-term, nightly monitoring of the recurrent nova U Scorpii. This nova, with a recurrence time of approximately ten years, last erupted in 1999. It is believed due for another outburst in 2009 -- Schaefer predicts the outburst will occur in 2009.3 +/- 1.0 year.

Schaefer is using this object as a test of a theory outlined in his 2005 paper on possible triggers for nova eruptions. It is hoped that long-term monitoring by AAVSO observers will provide rapid notification of outbursts and a photometric record of U Sco prior to and throughout the next outburst. Several ground and space-based observatories including the Hubble Space Telescope, Swift, and the Spitzer Space Telescope will be used to observe U Sco when it erupts, and your observations may be used to trigger these observations.

AAVSO observers are asked to add this object to their list of monitored objects. Both visual and CCD observers are asked to monitor this star for outbursts, and upon detection, to report observations as quickly as possible to the AAVSO. They are then asked to follow the outburst throughout the expected rapid rise and decline. CCD observers capable of detecting this star in reasonable time are asked to obtain occasional positive detections as a record of U Sco's behavior prior to outburst. U Sco is extremely faint outside of outburst, so positive observations will be difficult. "Fainter-than" observations are also valuable as they can confirm the object is not in outburst.

This object lies very close to the ecliptic, and solar interference is severe near conjunction. Observers are asked to obtain observations of U Sco as close to the Sun as possible, particularly when it is a morning object; outbursts typically last for only a few months, and it is possible that an entire outburst may be lost during conjunction.

Bradley Schaefer informs us that the object was marginally detected within the past week using the ROTSE IIIb telescope in Texas; U Sco was at a magnitude consistent with normal quiescence. Therefore it is unlikely that U Sco went into outburst and declined during the recent solar conjunction, and is still likely to go into outburst during the next two years. U Sco is currently a morning object, and observations made during its morning apparition are strongly encouraged.